It is known that in gearing it is desirable to provide for a certain number of clearances and in particular backlashes or play between the teeth of driving and driven wheels respectively. These clearances are necessary on the one hand because of the manufacturing tolerances and on the other hand because they avoid jamming of the two wheels during their rotation. At the same time, because of such backlashes, the driven wheel sometimes rotates jerkily. This is still further accentuated when it concerns the last wheel in a gear train including several.
In horology this problem is crucial to the extent that the user of a watch may note that one of the hands advances by jumps. Such may be the case for instance for the seconds hand when the latter is secured onto the last wheel of a gear train. If it concerns a watch without complications, the user has the impression that the seconds wheel rotates in an irregular fashion and, if it concerns a chronograph watch, that the displayed time lacks precision.
In order to overcome these difficulties, there has already been proposed, as described in Swiss patent CH 506 824, an arrangement providing friction on a rotating element in a chronograph movement. More precisely, there is arranged a spring under a chronograph wheel which is borne by a shaft to which it is fixed. This spring comprises an annular portion which is coaxial to such shaft as well as two diametrally opposed arms fastened at their free ends onto a bridge. It thus exerts an axial force on the chronograph wheel in order to prevent knocking of the chronograph hand.
However, this type of friction arrangement can only be employed if the wheel requiring braking is located directly above the bridge and on condition that one has available sufficient space between such wheel and the bridge in order to place such arrangement. Sometimes the wheel which one wishes to brake is itself placed above one or several other gear wheels. It is then impossible to employ the arrangement previously suggested.
There are also known braking arrangements employing magnetism. In this case a magnet, generally of platinum-cobalt, is secured for instance in the bridge placed above the wheel to be braked. Under the action of such magnet, the wheel is drawn towards the bridge and its axis rubs on the bearing situated in the bridge, the effect of which is to brake the wheel.
At the same time, platinum-cobalt magnets are expensive and this technique requires the employment of gear trains formed in a material having magnetic permeability.
As a consequence, the invention has as purpose to overcome the difficulties previously cited from the prior art by providing a braking arrangement for a gear wheel which is inexpensive and capable of being adapted to any type of gearing whatsoever.